It is common practice in jamming range gated radars to utilize a system which samples the frequency of the incoming pulse from an "enemy" radar and generates a pulsed signal having this frequency. This signal is then transmitted back to the "enemy" radar such that the inter-pulse spacings are gradually varied to deceive the radar as to the range of the target. One such system for accomplishing this is a system which utilizes the incoming pulse, amplifies it, delays it, and couples it back into the transmission line. The delay is produced by a delay circuit which is in a feedback loop and the delay is set such that the leading edge of the delayed signal meets the trailing edge of the preceding signal such that an essentially continuous CW signal is available at the output of the recirculating memory. This signal is then gated in an appropriate manner to provide the output pulses which convey false range information.
The system is exceedingly simple in that it reproduces a signal having the same frequency as that of the incoming pulse over a wide range of input pulse frequencies. However, one of the problems with this system is that the output signal amplitude fluctuates with the frequency of the incoming signal. This results because in the production of the CW signal the delay normally used is fixed. With a fixed delay, the effective recirculation loop length may not be a whole number of wavelengths of the particular input signal which arrives at the jammer, and there may be a phase shift between the leading edge of the delayed pulse and the trailing edge of the preceding pulse as recirculation occurs. If, for instance, a 180.degree. phase reversal occurs it is theoretically possible that one portion of the signal from the quasi-coherent memory will cancel another portion. Thus, the amplitude of the output signal from the quasi-coherent memory can theoretically go to zero. In actual practice, however, this exact phase relationship rarely, if ever, occurs. However, significant amplitude decreases do, in fact, occur at regular frequency intervals absent any adjustments of the effective length of the recirculation loop with input signal frequency.
Thus, the problem with prior art jammers utilizing recirculating memories is that in order to compensate for these reduced power problems, the jamming transmitter must have an inordinately high power rating.
One method of reducing amplitude fluctuation described in copending application Ser. No. 264,123, filed June 19, 1972 by Victor Trush and assigned to the assignee hereof is to phase shift the delayed signal to effectively lengthen or shorten the recirculation loop such that the phase difference between the signal at the leading edge of the delayed pulse and the signal at the trailing edge of the preceding pulse is reduced to no more than 45.degree.. This effectively adjusts the length of the recirculation loop to approximate a whole number of wavelengths of the incoming signal. The Trush apparatus accomplishes this by gating the incoming pulse through a "quasi-coherent" memory circuit which has an adjustable delay loop to recirculate the incoming pulse. The phase difference between the input signal and the delayed signal is then sensed and a predetermined phase shift is introduced into the recirculation loop to effectively lengthen or shorten the loop. After the required phase shift is inserted into the feedback loop, signals are recirculated to produce a CW signal of substantially constant amplitude. This CW signal is then applied to an amplifier such as a traveling wave tube (TWT) which is modulated in accordance with the variable inter-pulse spacing desired for deceptive range gate jamming. It will be appreciated that the system thus far described requires both a phase shifter and a phase detector as separate circuits.
It is the purpose of the subject invention to provide a single configuration invariant phasing network which functions both as phase detector and phase shifter. For purposes of the subject invention the term "configuration invariant" means that no changes are made to the circuit to switch it from its phase detecting mode to its phase shifting mode. Additionally, with the use of this circuit the switching system for providing the required phase shift is moved from its initial position between the aforementioned phase shifter and phase detector to a position between the hybrid phasing network and the TWT amplifier utilized in the quasi-coherent memory. In this hybrid system the input signal is applied to the phasing network which, in one embodiment, has four output terminals and four associated output lines coupled to a four position switch. Signals on these lines are phase shifted by 0.degree., 90.degree., 180.degree. or -90.degree. with respect to a feedback signal coupled back to the phasing network. Initially one of the lines is coupled through the four position switch to a traveling wave tube (TWT) amplifier to introduce the input pulse into the loop. It does not matter which line is chosen. A portion of the output signal from this amplifier is tapped off, delayed and fed back to a feedback input terminal of the phasing network. This feedback signal is mixed with the input signal such that the amplitudes of the signals on the four output lines are related to the phase difference between the feedback signal and the input signal. At this point the phasing network acts as a phase detector such that the relative amplitudes of the signals on the four output lines indicate the phase difference between the feedback signal and the input signal. These signals are rectified and the corresponding d.c. voltages are sensed at a logic circuit which then controls the four position switch to switch that line to the TWT which results in the feedback signal being in phase (or closest to being in phase) with the input signal. This effectively adjusts the length of the recirculation loop for the wavelength of the incoming signal.
The way this is determined is as follows: assume the feedback signal is found to be between 45.degree. and 135.degree. out of phase with the input signal, the four position switch switches the line which shifts the feedback signal by -90.degree. to the TWT. Thus the phase difference between the input and feedback signals is reduced to less than 45.degree.. This in effect adjusts the delay of the recirculation loop to the frequency of the incoming signal so that that which is recirculated will be in phase, or close to being in phase, with that portion of the signal which is amplified by the traveling wave tube. After the switch is set (after phase detection) the phasing network acts solely as a phase shifter, with the switch staying in the position set by the logic until the next input pulse arrives. Thus, after the first recirculation (the recirculation of a portion of the input pulse) the logic is inhibited and stays inhibited until the arrival of another input pulse is sensed. The hybrid phasing network thus performs first the function of a phase detector, and then the function of a phase shifter.
In this manner the output from the quasi-coherent memory is a wave train in which one portion of the wave does not vary in phase from another portion of the wave train by more than 45.degree.. It is therefore impossible for a large phase shift to occur in the output signal from the quasi-coherent memory and this memory therefore has a relatively flat amplitude response.
An additional advantage of combining the aforementioned phase shifting network and the aforementioned phase detecting network is a decreased loop loss by using one circuit performing the function of two circuits. Moreover, size and weight savings are realized by the single subject network. Additionally, since the jammer may be exposed to pulse trains from multiple radars simultaneously, there is an advantage in maintaining the high speed operation of the quasi-coherent memory with a short cycle time so that the subject jammer will be able to accommodate multiple incoming radar pulse trains. It will be appreciated that with each incoming pulse, the four position switch is set so that the phase error is minimized. The system can therefore accommodate a pulse from a first pulse train, set the switch, and transmit a return pulse with the prescribed delay and then accomodate an incoming pulse from a second incoming pulse train. The resetting of the quasi-coherent memory and the gating of the incoming pulses to the quasi-coherent memory is controlled by a conventional logic circuit and threshold detector such that the quasi-coherent memory can be made to operate on each distinct incoming pulse.
It is therefore an object of this invention to provide an improved jamming system operative against range gated radars.
It is another object of this invention to provide a single phasing network which accomplishes both a phase shifting and phase detection function.
It is a further object of this invention to provide an improved frequency memory in which output amplitude fluctuations are minimized.
It is yet another object of this invention to provide a memory for producing an essentially constant amplitude CW signal which has the same frequency as an incoming signal in which the memory employs a recirculation system utilizing a single phasing network which insures that any given portion of the CW signal varies by no more than 45.degree. in phase from any other portion of the CW signal.
It is a yet still further object of this invention to provide an improved phasing network and four position switch combination for use in a quasi-coherent memory in which information is tapped from the output lines of the phasing network to instruct a logic circuit to control the position of the four position switch thereby to inject the appropriate phase shift into the recirculation loop of the quasi-coherent memory.
These and other objects of this invention will be better understood in connection with the following description in view of the appended drawings in which: